WASHINGTON (CNN) -- A sharp increase in deadly accidents involving U.S. Marine Corps aircraft has forced a close look at possible causes, officials said Tuesday.

From October 2003 through September 2004, the Marines sustained 18 major accidents, including the deaths of 15 aviators and the loss of 21 helicopters and fighter planes.

Most of the accidents came during training missions in the United States. The others occurred during combat operations in Iraq and Afghanistan.

Those numbers mark the worst year for Marine Corps aviation safety in more than 10 years, according to Marine records.

More Marine aviators were killed in accidents over the fiscal year that ended September 30 than were shot down by enemy fire, according to Marine officials.

The accidents cited are those termed "Class A" -- mishaps in which at least one person died or at least $1 million in damage occurred to an aircraft.

Marine officials set a mishap goal of 2.9 Class A accidents per 100,000 flying hours, but the rate for the 2004 fiscal year ended at 5.3 accidents per 100,000 flying hours.

By comparison, the Marines sustained 11 major accidents in the previous period, or 2.9 major accidents per 100,000 flying hours.

Accidents included the collision of two Marine F-18 Hornet fighters over the Atlantic Ocean; the crash of a UH-1 helicopter that killed four people during a night training mission; and the collision of two F-18 Hornets over Oregon that killed the two pilots.

Marine officials denied that the increased pace of combat operations contributed to the high accident rate, saying it was due instead to a leadership problem in the aviation ranks.

"We are currently taking significant losses from a self-induced internal threat: non-combat mishaps," the commandant of the Marine Corps, Gen. Michael Hagee, wrote to his aviation commanders in July.

"Our peacetime training mistakes are significantly degrading our ability to prosecute the Global War on Terrorism," he said.

The officials said that recommendations to hold commanders and squadrons accountable are expected to be announced in January.

Some commanders have already been relieved. In one case, five commanders from a Marine squadron in Iraq were fired in October because of a high accident rate within that group.

One month into the new fiscal year, the Marines have sustained no major aviation accidents.